University of Vermont Extension
Department of Plant and Soil Science

Spring News
ArticleGROWING EXCELLENT ONIONS

Dr. Leonard Perry, Extension
Professor
University of Vermont

If
you’re a gardener, you may already be growing onions. If not, or
new to gardening, and you like to
use onions in cooking, consider growing your own. They don’t take
up much room, are an easy and
tough crop, and by growing your own you can get cultivars
(cultivated
varieties) not available at markets. There
are a few facts on onion growing which can make your onions
excellent, instead
of just good.

First,
you’ll need to choose which onions to grow.
Consider the color—they come in reds, yellows, and whites. Consider
the size, which often relates to
use, such as the small white pearl onions for stews or large ones
for
slicing. Consider if you want to eat and
use them fresh or store them, as some store much longer than
others. Of course consider taste—do you like sweet or
strong flavors?

Perhaps
most important culturally is their day length requirement. Onions,
depending on cultivar, require
various amounts of light in order to form bulbs. For northern
gardens look for “long day”
onions which require at least 14 hours of light daily in summer to
“bulb
up”. Or, you can choose “day neutral”
ones, for which day length doesn’t matter.

Next,
figure how you’ll start your onions.
They can be grown from seeds, plants, or “sets”. The latter are
small bulbs which you plant with
their pointed side up, tips just level with the soil surface, about
3 to 4
inches apart in a staggered spacing between rows. Contrary to what
you might think, smaller
sets are better as they are less likely to “bolt” than larger ones
(flower and
stop forming bulbs prematurely). Sets are often recommended as the
easiest
method.

You
also can start onions from purchased small plants, which are easy
and what many
do, but plants or sets offer you fewer choices than seeds. If
you’re starting from both sets and
plants, put them in different areas,
as the sets may be more prone to diseases.
Space plants in rows similar to sets.

Perhaps
the most variety is from starting plants yourself from seeds. Since
they are a long season crop, taking
over 3 months and often 4 months from sowing to harvest, it is best
to start
seeds indoors in the north. Seeds are
short-lived, so start with fresh ones each year.

Sow
seeds in flats mid-February to mid-March-- about 8 to 10 weeks
before planting
out. Sow them just below the soil
surface in rows and, when they are about 5 to 6 inches tall, thin to
one-quarter inch apart or transplant into individual cells in
6-packs. If tops droop, cut them back to about
4-inches high. Use a heating mat to keep
soil temperatures above 65 degrees (F)—75 to 85 degrees is best.

Whether
you’re planting sets, purchased plants, or those you started
yourself from
seeds, plant out about 2 to 4 weeks before the last usual frost date
in your
area. For northern gardens, this may
mean planting out late-April to mid-May.
To avoid problems and get the best crop, don’t plant them in the
same
area onions or their relatives have grown the last couple years, nor
where
legumes (peas and beans) have recently been grown. Onions grow best
in a well-drained, loose soil
with lots of organic matter. A sandy
loam with additional compost is ideal.

If
starting your own seedlings, fertilize with a half-strength
fertilizer when you
thin or transplant into cells. Then, in
late spring after you plant out, “sidedress” (apply fertilizer along
the side
of the rows) with compost. If leaves
aren’t a dark green, you can use an organic fertilizer such as fish
emulsion
once a month. A key is not to over fertilize
with nitrogen, as this will result in great leaves but small bulbs.
Onions do need sufficient potassium, or
they’ll have hard “necks” (the stem above the bulb) and bulbs won’t
store as
well.

Usually
onions have no problems, except for weeds and perhaps for onion
maggots. To discourage this pest, don’t plant too
early in spring, and use row covers for the first few weeks to
discourage the
flies laying eggs. Plants don’t compete
well with weeds, so keep them weeded, particularly when young.
Mulching after planting with straw or grass
clippings reduces weeding.

Another
key to an excellent crop is to keep the soil moist during the first
couple
weeks after planting out. Then, at the
end of the season, the opposite applies.
As plants mature—no more new leaves and tops fall over—reduce
watering,
and pull soil away from the tops of the bulbs.
This encourages them to go dormant and start the drying process.

Plants
should be dormant before harvest, or the bulbs won’t store well.
The sign to harvest is when most the tops
have fallen over. Harvest when the
weather is dry, remove loose soil, then “cure” onions-- place in a
warm and
airy place, out of direct sun, for 3 weeks before storing. Then,
you can cut tops back to an inch above
the bulb, unless you want to leave them on for braiding many
together as you
often see in farmer’s markets.

Store
bulbs cool (32 to 40 degrees F is ideal) and dry, with good air
circulation. You can do this in mesh
bags, or shallow boxes with newspaper layers between bulbs. Check
periodically to make sure they aren’t
getting soft spots or rots. Generally
the large, white, sweet onions only store for a few weeks to a
couple months
from harvest. Those with yellow or brown
skin, with hard necks and pungent flavor, store for months. Those
grown from sets tend not to store as
long as those grown from plants. Keep
stored onions away from apples and tomatoes, as these give off a gas
(ethylene)
which makes onions sprout. Onions are
easily frozen, too. Merely peel, chop,
and loosely pack in freezer bags.